1. Field of the Invention
The present invention relates to a common mode choke coil, and more particularly to a common mode choke coil having edgewise windings of a rectangular insulated wire.
2. Description of the Related Art
In recent years, since the miniaturization and enhanced performance of an electronic apparatus have been strongly demanded, a common mode choke coil used in a line filter is required to be downsized and improve in performance. Conventionally, a common mode choke coil mostly uses windings of a round insulated wire but recently uses edgewise windings of a rectangular insulated wire increasingly because the edgewise winding has the following advantages over the winding of a round insulated wire. Firstly, the edgewise winding can better achieve higher performance, higher efficiency, miniaturization and lower-profile due to its larger conductor occupation ratio. Secondly, the edgewise winding has a smaller stray capacity and therefore can realize better frequency characteristics. And thirdly, the edgewise winding does not require a process of winding a wire on a bobbin and is easier to assemble, resulting in an easier automation of the manufacturing process. The edgewise winding is formed using a special tool such that a flat rectangular insulated wire, shaped like a foil, is wound with a bottom flat surface of a winding layer stacked on a top flat surface of the previous winding layer (refer to Japanese Patent Publication No. H04-75303).
A coil device using the edgewise winding described above is disclosed, for example, in Japanese Patent Publication No. H10-241955 (refer to FIG. 6 therein), which comprises: a magnetic core; two edgewise windings formed of a rectangular wire, each having two lead wires, and provided around legs of the magnetic core; and a resin case composed of a case body and a case cover, and housing the edgewise windings. In the coil device, at least one of the lead wires of each of the edgewise windings is inserted in a groove formed inside the case body, and a harness lead wire is inserted in a hole formed in the lead wire and soldered to the lead wire.
A surface mount type choke coil is disclosed, for example, in Japanese Patent Publication No. H08-236364 (refer to FIG. 3 therein), which is structured such that a rectangular insulated wire is wound around a magnetic core leg with its both terminations leading out respectively at both opposing sides of the core, and the both terminations have their insulation peeled off thereby constituting a pair of flat connecting portions.
Also, a surface mount type small-size coil component is disclosed, for example, in Japanese Patent Publication No. H11-345721 (refer to FIG. 1 therein), which comprises two or more windings, a bobbin, and a magnetic core of magnetic material such that at least one winding is an edgewise winding of a rectangular wire and stacked on another winding of flat type.
And, a coil device with insulation assured between terminals of windings is disclosed, for example, in Japanese Patent Publication No. H08-264338 (refer to FIG. 1 therein), which comprises a plurality of edgewise windings of a rectangular wire wound on a cylindrical bobbin. In the coil device, a partition perpendicular to the cylindrical axis of the bobbin is provided at each of both ends of the bobbin and between each of the windings, at least a portion of a magnetic core is inserted in the bobbin, terminal pins are provided at the ends of the rectangular wires of the edgewise windings, and a pair of plates opposing each other in parallel and adapted to hold the terminal pins are disposed along the cylindrical axis of the bobbin.
When any of the edgewise windings described above is wound on a bobbin, especially for a common mode choke coil, the following must be taken into consideration. Firstly, the starting and finishing ends of the winding must not be set in contact with or close to each other for preventing increase of distributed capacity due to winding. Secondly, windings of common mode must be wound on one bobbin for increasing the winding factor. And thirdly, withstand voltage between lines must be increased because currents having opposing polarities flow in windings.
Consequently, typically in a common mode choke coil of bobbin type, one bobbin is used while maintaining insulation between lines, and also the cylindrical axis of the bobbin is directed parallel to a mount board. As described above, the winding factor is increased by providing windings of common mode on one bobbin, which is considered in terms of a number of bobbins or workability of winding process. On the other hand, the winding factor can be discussed essentially from the aspect of a magnetic core such that it is not necessarily appropriate to provide windings on one bobbin in trying to effectively provide windings around a magnetic core thereby achieving a maximum inductance for the same size.
Under the circumstances described above, there is conventionally provided a common mode choke coil using edgewise windings (refer to FIG. 5) for increasing winding factor, increasing current capacity, realizing downsizing and lower profile, and for reducing distributed capacity of winding. The common mode choke coil has two edgewise windings 6 wound in common mode and provided respectively around both legs of a UU core 5 for increasing winding factor. The common mode choke coil usually has its bobbin laid horizontally as shown in FIG. 5. One reason for the horizontal arrangement is that though the rectangular wire has a smaller distributed capacity of winding than the round wire therefore showing good frequency characteristics of impedance, if the starting and finishing ends of the winding are brought in contact with or close to each other, the distributed capacity is increased resulting in loss of the advantage of the edgewise winding, which does not happen with the common mode choke coil shown in FIG. 5 that can be attached on a board with its starting and finishing ends kept from getting in contact with or close to each other. Another reason is that since the core 5 is held by a metal spring 3 which is cantilevered and therefore has a weak holding force, the core 5 is apt to move out of place when subjected to mechanical vibration resulting in deterioration of performance, which can be restrained by the horizontal arrangement for a lower profile.
FIG. 6 shows another conventional common mode choke coil which has its bobbin arranged vertically. The common mode choke coil is advantageous in requiring a smaller mounting space than the common mode choke coil of FIG. 5 but has a disadvantage that lead wires 8, when bent, are positioned close to or even in contact with edgewise windings 6, thus causing difficulties with workability. Also, in this common mode choke coil, the magnetic core, bobbin and windings are not fixed adequately against mechanical and electrical vibration, which limits its usage to special applications.
In any of the common mode choke coils shown in FIGS. 5 and 6, since the bobbin 9 is not fixed to the legs of the core 5 when attached therearound, the bobbin 9 must be fixed thereto by adhesive or the like. Also, since the edgewise windings 6 are not fixed to the bobbin 9 when attached therearound and cannot be fixed mechanically, the edgewise windings 6 must be adhesively fixed thereto. Fixing the bobbin 9 and the windings 6 by adhesive separately after attaching process results in lowering the workability and also possibly deteriorating the withstand voltage and the frequency characteristics due to adhesive applied.